Flexible polymeric films are widely used as packaging materials for a vast range of goods. A variety of synthetic polymers may be used as packaging materials, for example films made from synthetic polymers such as polyolefins (e.g. polyethylene and/or polypropylene), polystyrene and/or polyesters; and/or films made from natural polymers such as cellulosic materials and/or other biopolymers (e.g. polylactic acid).
One particular packaging use for such films is as an overwrap for a variety of goods, for example for cigarette packets, video tapes, foodstuffs etc., the films being sealed tightly over the goods. However, the very properties which confer desirable properties on the films as packaging materials, for example high strength and tear resistance, make such packages difficult to open because the films of which they are made are difficult to tear.
In order to facilitate the opening of such packages, so-called “tear tapes” have been provided which consist of a narrow strip of a polymeric film adhered to the internal surface of the packaging film, a tab of the tear tape being left free on the outside of package to facilitate its opening. The package can then be opened by pulling the tear tape through the packaging film.
Another method proposed to open a package wrapped in film is to provide inherent in the film one or more lines of weakness on the film. Two lines are generally preferred to define a film strip of sufficient width so a free tab end attached thereto can be easily grasped and pulled. To open the pack the tab is pulled to tear the film completely around the pack along both lines of weakness to remove the strip which severs the film and hence opens the pack.
Lines of weakness on the film have advantages over use of a separate tear tape as they are integral to the film and do not require additional processing steps or extra material. A tear tape increases the thickness of the film where it is added resulting in an uneven film surface which can reduce the seal integrity of the pack. The mechanical properties desired for a separate tear tape are different from those required for an overwrap film. So the tear tape and film are often made from different, incompatible materials which can make it difficult to provide a good heat seal in regions of the pack where the overwrap film is adjacent the tear tape.
Lines of weakness can be made by any suitable methods for example by mechanical scoring (e.g. with a controlled blade or roller to applies pressure to the film surface), by cut or scored perforations, by non contact means such as laser or other radiation to ablate and/or burn off a thickness of film (e.g. as described in U.S. Pat. No. 3,909,582; U.S. Pat. No. 5,630,308 (both American Can) or U.S. Pat. No. 5,010,325 and U.S. Pat. No. 5,010,231 (both LPF)); by use of a laser to scramble orientation along a line on the oriented film without ablating the film surface (as described in the applicant's co-pending application PCT/EP02/00075); and/or by the methods described in WO 01/15594 (Hoescht) or WO 01/94098 (University of Warwick).
Although tear tapes and tear lines can be a good and efficient way of opening such packages, it can be difficult to initiate the tear. To this end a notch or tab is typically cut on the sheet around the tear tape or line so that the tab can be easily grasped and pulled to propagate a tear along the weakened line or tear tape to completely sever the wrapped film around the package. Such tabs are generally U shaped with sides parallel to the tear line or V shaped where the sides taper towards the free end. However any shaped tab may be used with the present invention such as those tabs described in the applicant's co-pending British patent application (applicant's case reference 31.14 GB) filed the same day as the present application. The contents of this co-pending application are incorporated herein by reference.
Before being used to wrap an article the film web is cut across the width of the web in the transverse direction (TD) to form sheets which are used to wrap each article. For convenience this cut is referred to herein as the “sheet line”. The tear lines or tear tape are arranged on the sheet to run in a perpendicular direction, the machine direction (MD). The tab will be cut in the web to lie across the sheet line, i.e. the tab will protrude from the leading edge of one sheet and thus form a correspondingly shaped in the trailing edge of the following sheet. The tab will also be positioned to straddle the tear tape, or tear line(s). To prevent the tab being ripped off the film in the process of opening the pack, the sheet line has an uncut gap (i.e. where the web is still attached) which is in register with the position of the tear lines. This can be achieved by any suitable means for example the slitting knife may have a notch positioned in register with the tear lines or tear tape.
To wrap a pack (e.g. cigarette carton) or articles (e.g. line of biscuits wrapped in a tube) there will be some overlap between the trailing and leading edges of a sheet as it wraps around the article and this overlap of film is where the film is heat sealed together for example by being offered up to a heated metal bar. When the overlapped film is sealed together the film initially forms a tube around the article. The open ends of the tube can then folded around the article and heat sealed together to form an air tight seal enclosing and protecting the article. If the tab is larger that the overlap region the corresponding notch cut in the sheet trailing edge will protrude beyond the sealed region of overlap and seal integrity will be lost. Yet it is desired that the overlap region is as small as possible consistent with a good heat seal for the following reasons. A large area of overlap for each sheet is expensive as it wastes film but also the heat sealed overlap is less transparent (for clear films), may spoilt the visual look of the wrapping and/or may be difficult to print. For this reason the sheet line is positioned about half way up the tab so that the overlap region is not too large.
However problems have been found with the use of tabs and the overlap region on a package. If the region of overlap is heat sealed the free end of the tab will be difficult to find as it will be sealed to the underlying film in the overlap region. Too strong a seal in the overlap region can also pose problems as then it can be difficult for a tear to propagate through the region of overlap. On the other hand if the overlap region around the tab is left unsealed or the seal strength in the overlap region is reduced to aid tearing this will compromise the seal integrity of the pack which is also undesirable.
Thus it is desirable to provide an improved means for wrapping an article to form a package which can be readily opened by hand, optionally whilst retaining good seal integrity of the package before opening.